In suspension systems between vehicle bodies and the wheels of the vehicle, use is conventionally made of a suspension system provided for each wheel and consisting of a spring and a shock absorber--or damper--connected in parallel between body and wheel.
The body forms a first mass which is to be sprung, and the wheel, which comprises tyre and hub, forms a second mass which is considered as an unsprung mass. However, the wheel does possess a certain springing in the tyre, but in this context is designated as the unsprung mass.
The suspension system is intended to damp the transmission of road unevenness to the body and, at the same time, ensure that the tyre abuts against the road surface for retaining road-holding properties. These objects are in conflict with one another, and so conventional suspension systems are designed for the best possible compromise between these properties.
Two resonance ranges are critical for such suspension systems. In a first resonance range, the body will assume a state of resonance if the disturbing frequency lies typically within 0.8-4 Hertz (Hz), normally about 1 Hz. In this first resonance range, insulation of disturbing impulses from the road surface or carriageway is impeded. In a second resonance range, the wheel will assume a state of resonance if the disturbing frequency lies typically within 10-15 Hz, normally about 10 Hz. Insulation of the disturbing impulses of the road surface and road-holding properties also deteriorate in this resonance range.
The damper is intended to reduce those disturbing impulses which are transmitted to the body in these resonance ranges. However, the choice of a fixed, so-called passive damping constitutes a compromise between, on the one hand, increasing transmission of disturbing frequencies outside of the resonance range With reduced resonance oscillation as a consequence, and, on the other hand, reduced transmission of disturbing frequencies outside of the resonance range, with increased resonance oscillation as a result. For example, relatively hard damping may damp the natural resonance of the body, while disturbing frequencies outside the resonance range will have increased transmission. Conversely, a gentler damping will provide satisfactory damping of disturbing frequencies outside the resonance range. While the body or the wheel will more readily assume a state of powerful resonance. In disturbing frequencies within each respective resonance range.
In order further to improve suspension systems, both semiactive and active suspension systems have been proposed in the art. A feature of the semiactive suspension systems is that the damping in the dampers is modified in response to the direction and amplitude of the disturbing frequency, without any external energy being supplied to the damping operation. According to the active systems, external energy is supplied to the damping work in response to the direction and amplitude of the disturbing frequency, and possibly also related to the distance between the body and the wheel.
Both the semiactive and the active suspension systems have hitherto demonstrated that the transmission of disturbing impulses within the resonance ranges, in particular the resonance range of the wheel, has been excessively high. In order that the active suspension systems at least theoretically be capable of giving low transmission within the resonance range of the wheel, extremely complex steering systems with advanced transducers and indicators of high resolution have been necessary.
There are prior art solutions for governing the natural resonance of the wheel, so-called "wheel-hop". For example, Patent Application DE,A, 2217539 discloses a damping mass connected to the wheel and controlled in response to the movement of the wheel and the effect of the suspension system, the damper and the spring. A similar solution is disclosed in Patent Application EP,A,-200384 and may also be found reproduced as a principle in the Annual publication "Bosch Kraftfahrtechniches Taschenbuch" in the section "Federung". Common to all of these solutions ms that the resonance damping mass is included in a system which is parallel with and discrete from the damper and the spring, whereby considerable space must be reserved for this system for damping of wheel-hop.